The present invention relates to a paper feed device employed in a printing apparatus for automatically feeding a printing paper, more particularly to a device for feeding a printing paper to a platen for a predetermined amount and for operating a release mechanism for releasing a paper bail having been brought into contact with the platen so that a paper is smoothly fed between the platen and the paper bail.
Conventionally, a printing apparatus such as a typewriter which is equipped with an automatic paper feed device in which a printing paper is fed to a platen by an operation of a paper insert key provided on the printing apparatus and by releasing operation of a paper bail which is usually pressed to the platen by a release mechanism which is provided with an intermittent gear, release cam, and so forth from the platen, whereby the printing paper is smoothly fed between the platen and the paper bail has been used.
Generally, in the automatic paper feed device, for example, when a diameter of the platen is approx. 36 mm, the paper feed length necessary for the automatic paper feed operation is approx. 75 mm. When the printing paper is fed for the paper feed length, the rotation angle of the driven gear disposed at the platen is approx. 220.degree., for example. In addition, the paper bail is arranged to be placed in a release position where the paper bail is released from the active position in which the paper bail and the platen are brought into contact with each other until the printing paper is fed for a predetermined length and a leading end of the printing paper arrives in a position higher than the paper bail just after the automatic paper feed operation is started so that the printing paper can be smoothly fed between the paper bail and the platen.
To do that, a release cam is provided for an intermittent gear which is arranged to be engaged with the driven gear as a release mechanism of the paper bail, thereby rocking a paper bail arm on which the paper bail is supported.
The driven gear is normally made with the nearly same diameter as that of the platen. The driven gear is rotatable driven by a line feed motor through an intermediate gear. There are needs for increasing the degree of freedom in designing the paper feed device by freely selecting the speed reduction ratios of the line feed motor, driven gear, and intermittent gear and the diameter of the intermittent gear, thereby reducing a size thereof.
When the size of the intermittent gear becomes small, the speed reduction ratio of the intermittent gear against the motor gear which drives the driven gear becomes small. Thereby, the rotation angle after the paper feed is started until it is ended becomes large. The timing on which the paper bail is moved from the release position to the contact position becomes much faster than that on which the automatic paper feed operation is ended and thereby the printing paper being fed tends to be disturbed by the paper bail.
On the other hand, it is possible to take a countermeasure for decreasing the diameter of the driven gear in accordance with the size of the intermittent gear, because the speed reduction ratios from the line feed motor to the driven gear increases. Therefore, the size of the line feed motor becomes large.
Accordingly, in the conventional automatic feed device described above, the although the speed reduction ratios from the line feed motor to the driven gear can be set to larger values, the release mechanism including the intermittent gear becomes large, resulting in decreasing the degree of freedom in designing the device.
To solve such problems, it may be considered that a release mechanism having a tailed release cam is employed, as shown in FIGS. 1(A) and 1(B). As shown in FIG. 1(B), even when the driven gear 61 is turned for approx. 217.degree. in the automatic paper feed state and the intermittent gear 62 is remarkably turned for approx. 335.degree., the tail portion 60a can securely delay the moving timing on which the paper bail 63 is moved to the contact position.
However, in the release mechanism having the tailed release cam as shown in FIGS. 1(A) and 1(B), a shape of the release cam becomes complicated and thereby an operability and an durability thereof decrease.